Perhaps comparing the issue to animals is "missing the point" or "trivializing the issue" but as I have no experience with human loved ones or patients that's all I have to go by (and even though euthanasia with pets is well accepted, there are people who don't agree with it on religious or personal grounds). But ya, the hardest thing I had to witness was from a few months ago. There was a 15 year old Shih Tzu who was presented for not eating for 3 days. We did blood work and discovered her values were all over the place, most notably signs that typically lead to pancreatitis and (more importantly) kidney failure. Her physical condition in general was deteriorated with blank eyes, a dried out nose that she could barely breathe through, and was generally non-responsive to her environment. The owner didn't want to put her down, but even with attempted treatment she didn't respond to it. Kidney failure is a disease of constant degradation, so it's more or less terminal. She still didn't eat and could barely lift her head. The owner still wanted to try, so she left her with us to put her on IV fluids. All that dog did was scream, flail around on her side (sometimes hitting the cage wall repeatedly), wouldn't swallow any food or water placed in her mouth, and just not be a happy camper (even with the owner around). The poor dog didn't even respond to voice or physical contact, even if being carried it seemed like she would be sleeping with her eyes open and then suddenly "snap awake". It took several days for the owner (who was unsure if it was the right choice) to put her down. There couldn't have been a happy ending for that dog. If left alone, she would've just wasted away slowly.
I know that "euthanasia" and "assisted suicide" are two different terms (and switching from animal to human cases is opening up a can of worms), but the reasoning is the same. People want to die with dignity and without suffering. To many people, that dignity doesn't involve constant pain, not doing things that can make you happy, or putting a constant strain on your loved ones. Of course assisted suicide needs to be heavily regulated and never forced on an individual or family, but an option needs to be there. I've heard people consider assisted suicide as an "easy way out" or want people to "fight it out", though I don't agree with those viewpoints being imposed on people or families. There's no pay-off to fighting something that can't be meaningfully won (at least the way I see it). Doctors hate talking about assisted suicide, even if what they're doing is technically that (there are a few different names for it, but I heard of a program where they drug up a patient to the degree where they feel nothing and just waste away after a few days/weeks). There has to be more open discussion among the medical communities.
So yes, I support it.